1. Field of the Invention
The present invention generally relates to duplexers, and more particularly, to a duplexer having surface acoustic wave filters formed on one substrate.
2. Description of the Related Art
Antenna duplexers each having two surface acoustic wave (SAW) filters are used for 800 MHz band portable telephone devices and mobile communication devices today. These SAW filters employed in antenna duplexers are required to have high power durability to perform stable operations and to achieve long lives. Accordingly, the two filters of each duplexer have normally been formed with ladder filters that have SAW devices connected in a ladder-like fashion and exhibit relatively high power durability.
As there has been an increasing demand for smaller-sized portable telephone devices and mobile communication devices, SAW filters that can be relatively easily made smaller in size are preferred for antenna duplexers used for 1.9 GHz band mobile communication systems (such as Personal Communications Services (PCS) for the North American market, for example).
To produce smaller-sized antenna duplexers is expected whether it is for an 800 MHz band device or a 1.9 GHz band device. A smaller-sized antenna duplexer can be produced by forming two SAW filters on one substrate (as disclosed in Japanese Patent Application No. 2002-237739, for example).
However, two SAW filters formed on a small substrate in an attempt to produce a smaller device cause problems such as a decrease in the degree of stop-band suppression and an increase in crosstalk between transmission signals and reception signals. These problems are conspicuous especially in 1.9 GHz band antenna duplexers than in 800 MHz band antenna duplexers, and for this reason, it has been difficult to produce 1.9 GHz band antenna duplexers utilizing SAW filters.
So as to eliminate the problem of crosstalk between transmission signals and reception signals, two SAW filters are formed on two separate substrates to produce an antenna duplexer.
However, the SAW filters should preferably be formed simultaneously on one substrate to produce an antenna duplexer. One of the reasons for this is to realize a small-sized antenna duplexer. The other reasons are as follows. When two SAW filters are formed on two separate substrates, the center frequencies of the two SAW filters fluctuate with variations in the electrode film thickness and electrode finger width between process batches. As a result, the gap between the center frequencies fluctuate by twice as much as the fluctuation of the center frequency of each SAW filter, causing large variations in the characteristics of antenna duplexers. When two SAW filters are formed simultaneously on one substrate, on the other hand, the center frequencies of the two SAW filters are not affected by variations in the electrode film thickness and the electrode finger width. Accordingly, the gap between the two center frequencies does not vary greatly, and antenna duplexers with desired filter characteristics can be obtained. As described above, the characteristics of each antenna duplexer are greatly affected by the gap between the center frequencies of the two SAW filters. It is therefore preferable to form two SAW filters simultaneously on one substrate to achieve a higher yield.
As already mentioned, there are market demands for higher-frequency antenna duplexers and smaller-sized antenna duplexers. If these demands are to be met, however, the crosstalk between transmission signals and reception signals increases. Therefore, it has been very difficult to achieve good filter characteristics.